Extensive research for improving power conversion efficiency has been actively performed ever since an organic thin layer solar (photovoltaic) cell having power conversion efficiency of about 1% was developed by Tang in 1986 (C. W. Tang, Appl. Phys. Lett. 48: 183, 1986). Unfortunately, such research is currently at a standstill. A pioneering improvement in power conversion efficiency was achieved in early 2000, which has recently been put to practical use.
Since organic materials considered as a p-type semiconductor form excitons upon the absorption of photons and have low electron mobility, an organic thin layer photovoltaic cell must be constructed through junctions between p-type and n-type semiconductors. When an organic thin layer photovoltaic cell using fullerene having high electron mobility was introduced for the first time in 1993, such a device did not come into the spotlight due to its extremely low power conversion efficiency (N. S. Sariciftci, et al., Appl. Phys. Left. 62: 585, 1993; J. J. M. Halls, et al., Appl. Phys. Lett. 68: 3120, 1996). A photovoltaic device having power conversion efficiency of about 2.5%, which was fabricated by blending poly(3-hexylthiophene (P3HT) as a conductive polymer and fullerene (S. E. Shaheen, et al., Appl. Phys. Lett. 78: 841, 2001) as an electron transporting material, has been reported. Further, various attempts have been made to develop organic thin layer photovoltaic cells using the above mixture. A photovoltaic device having a power conversion efficiency of about 3.5% was developed in 2003 (F. Padinger, et al., Adv. Funct. Mater. 13: 85, 2003). Thereafter, an organic thin layer photovoltaic cell having a conversion efficiency of about 5% was developed by changing a blend ratio of P3HT and fullerene, and improving electron transportation rate through heat treatment (W. Ma, et al., Adv. Funct. Mater. 15: 1617, 2005).
It has been found that such a rapid improvement in power conversion efficiency is due to several causes, for example, an better formation of a bicontinuous phase (i.e., interpenetrating network) by changing a composition ratio of the composite, a morphological modification of the composite through heat treatment, an increase in hole mobility by improving P3HT crystallization, an enhanced adhesiveness between an active layer and a metal electrode, etc. The continuous improvement of composite characteristics makes possible to develop an organic thin layer photovoltaic cell having a power conversion efficiency of about 6%.
As described above, the organic thin layer photovoltaic cell using polymer has been currently fabricated by applying a mixture of P3HT with high hole mobility and fullerene with high electron mobility on a transparent conductive substrate such as ITO substrate. To achieve high power conversion efficiency, it is indispensable to use a conductive polymer, PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate)], as a charge extraction layer and decrease contact resistance between a photovoltaic active layer and a metal electrode due to ohmic contact therebetween. Such use of a polymer:organic material blend has an advantage in that a device can be easily fabricated by mixing two kinds of organic semiconductors in a solvent by means of high processability of polymer and simply spin-coating the resulting mixture on a substrate. Thus, a fabricated organic semiconductor has been regarded as a future-oriented photovoltaic cell due to its simplicity, flexibility and low unit cost of production.
However, since such an organic semiconductor actually shows semiconductor characteristics, it has relatively high electric resistance. Further, it has been reported that the larger the device area is, the higher the series resistance is (B. Kippelen, et al., Appl. Phys. Lett. 89: 233516, 2006). Based on the above, there is a problem in that in a large area device, lateral contribution of series resistance which influences each other at different parts within the device being connected by the same charge transporting layer is increased. As such, total series resistance of the device is increased in proportion to the increased active area, which results in reducing the charge generation and power conversion efficiency.
Thus, the present inventors have endeavored to overcome the above problems of the prior art and found that if the device is divided into smaller size subcells by introducing device partitioning walls made of an insulating material inside the device during the fabrication of an organic photovoltaic device, each of the partitioned subcells works independently of each other. As such, their lateral contribution of series resistance does not mutually influence, which results in improving the power conversion efficiency of the partitioned device through the lowering of series resistance applied to the inside thereof.